Areva MiCOM P441, Техническое руководство

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Application Notes P44x/EN AP/E33
MiCOM P441/P442 & P444 Page 103/220
Case 2:
Source Impedance = 230
2
/ 3000 = 17.63
Ω
Line Impedance = 48.4
Ω
Fault current seen by relay = (230000 /
√
3) / (17.63 + 48.4)
= 2011A
The overcurrent setting must be in excess of 2251A. To provide an adequate safety margin
a setting
≥
120% the minimum calculated should be chosen, say 2800A.
3.2 Teed feeder protection
The application of distance relays to three terminal lines is fairly common. However, several
problems arise when applying distance protection to three terminal lines.
3.2.1 The Apparent Impedance Seen by the Distance Elements
Figure 62 shows a typical three terminal line arrangement. For a fault at the busbars of
terminal B the impedance seen by a relay at terminal A will be equal to :
Z
a
= Z
at
+ Z
bt
+ [ Z
bt
.(I
c
/I
a
) ]
Relay A will underreach for faults beyond the tee-point with infeed from terminal C. When
terminal C is a relatively strong source, the underreaching effect can be substantial. For a
zone 2 element set to 120% of the protected line, this effect may result in non-operation of
the element for internal faults. This not only effects time delayed zone 2 tripping but also
channel-aided schemes. Where infeed is present, it will be necessary for Zone 2 elements
at all line terminals to overreach both remote terminals with allowance for the effect of tee-
point infeed. Zone 1 elements must be set to underreach the true impedance to the nearest
terminal without infeed. Both these requirements can be met through use of the alternative
setting groups in the P441, P442 and P444 relays.
Zbt
A
Zat
Ia B
Ib
C
Zct
Ic
P3075ENa
Va = Ia Zat + Ib Zbt
Ib = Ia + Ic
Va = Ia Zat + Ia Zbt + Ic Zbt
Impedance seen by relay A = Va
Ia
Za = Zat + Zbt + Ic Zbt
Ia
FIGURE 62 - TEED FEEDER APPLICATION - APPARENT IMPEDANCES SEEN BY RELAY
3.2.2 Permissive Overreach Schemes
To ensure operation for internal faults in a POP scheme, the relays at the three terminals
should be able to see a fault at any point within the protected feeder. This may demand very
large zone 2 reach settings to deal with the apparent impedances seen by the relays.
A POP scheme requires the use of two signalling channels. A permissive trip can only be
issued upon operation of zone 2 and receipt of a signal from both remote line ends. The
requirement for an 'AND' function of received signals must be realised through use of contact
logic external to the relay, or the internal Programmable Scheme Logic. Although a POP
scheme can be applied to a three terminal line, the signalling requirements make its use
unattractive.